Substations in high- and medium-voltage power networks include primary devices such as electrical cables, lines, bus bars, switches, power transformers and instrument transformers, which are generally arranged in switch yards and/or bays. These primary devices can be automated via a Substation Automation (SA) system. The SA system can include secondary devices, among which Intelligent Electronic Devices (IED), responsible for protection, control, and monitoring of the primary devices. The secondary devices can be assigned to hierarchical levels, i.e. the station level, the bay level, and the process level, the latter being separated from the bay level by a so-called process interface. The station level of the SA system includes an Operator Work Station (OWS) with a Human-Machine Interface (HMI) and a gateway to a Network Control Centre (NCC). IEDs assigned to the bay level, also termed bay units or protection IEDs, in turn can be connected to each other as well as to the IEDs assigned to the station level via an inter-bay or station bus for exchanging commands and status information.
Secondary devices assigned to the process-level can include sensors for voltage (VT), current (CT) and gas density measurements, contact probes for sensing switch and transformer tap changer positions, and/or actuators (I/O) for changing transformer tap positions, or for controlling switchgear like circuit breakers or disconnectors. Exemplary sensors such as non-conventional current or voltage transformers can include an Analogue to Digital (AD) converter for sampling of analogue signals. Each sensor can be connected to the bay units via a dedicated or intra-bay process bus, which can be used as the process interface, which replaces the conventional hard-wired process interface. The latter connects conventional current or voltage transformers in the switchyard to the bay level equipment via dedicated Cu wires, in which case the analogue signals of the instrument transformers are sampled by the bay units.
A communication standard for communication between the IEDs of a substation has been introduced by the International Electrotechnical Committee (IEC) as part of the standard IEC 61850 entitled “Communication networks and systems in substations”. For non-time critical messages, IEC 61850-8-1 specifies the Manufacturing Message Specification (MMS, ISO/IEC 9506) protocol based on a reduced Open Systems Interconnection (OSI) protocol stack with the Transmission Control Protocol (TCP) and Internet Protocol (IP) in the transport and network layer, respectively, and Ethernet and/or RS-232C as physical media. For time critical messages, such as trip commands, IEC 61850-8-1 specifies that the Generic Object Oriented Substation Events (GOOSE) build directly on the Ethernet link layer of the communication stack. For very time-critical signals at the process level such as measured analogue voltages or currents, IEC 61850-9-2 specifies that the Sampled Values (SV) protocol also build directly on the Ethernet link layer. Hence, part 9 of the standard defines a format to publish, as multicast messages on an industrial Ethernet, digitized measurement data from current or voltage sensors on the process level. Such SV or other process data may be transmitted over an inter-bay process bus, making the transmitted information available to neighboring bays. For instance for cost effective setups such as in medium or low-voltage substations, the inter-bay process bus and the station bus can be merged into one single communication network. In this case, the communication network can be used an inter-bay process bus that transmits, in addition to the process data, command, status, and report related messages otherwise exchanged via a dedicated station bus.
In the context of Substation Automation, redundant protection functionality is used to achieve higher availability and reliability. For transmission and sub-transmission substations, full redundancy of protection functionality can be achieved with two independently installed and wired Intelligent Electronic Devices (IED) per bay. On the other hand, for medium- and low-voltage as well as industrial customer substations, a single IED protects the bay and therefore no redundant protection functionality is available. In the latter case, the IEDs represent a single point of failure for the entire bay.
Several approaches have been proposed to handle the failure of an IED. A hot-hot or hot-standby architecture is the classical approach for bays including two IEDs. In a hot-hot architecture, both IEDs are running in parallel. For a hot—standby architecture, the standby IED is taken into active use when the hot IED fails. Both approaches are conventionally realized by hard-wiring the inputs and outputs of both IEDs to the respective CTA/Ts (sensor input) and breaker actuators (I/O).
According to the patent application EP 1976177, in Substation Automation systems, the mean time to repair is reduced by the remote reconfiguration and start-up of a replacement or spare Intelligent Electronic Device (IED), allowing more time for the maintenance personnel to repair an inactive or faulty IED. The time required for the actual repair is irrelevant with respect to system availability as long as the repair time is short as compared to the IED failure rate. Therefore the remote configured spare IED leads to nearly the same availability as a hot—standby configuration, but without the need for doubling all the essential IEDs—where only one spare online IED is needed for each set of IEDs of the same type connected to the same station bus and process bus.
The patent application WO 2008/040263 discloses a first redundant protection system with three main protection IEDs plus one spare IED. The main IEDs are connected via dedicated “local process buses” and the spare IED is connected via an “inter-bay process bus” to respective protected objects. The buses transmit analogue values or digital sampled values, with possibly higher sampling rates on the local process buses than on the inter-bay bus. WO 2008/040263 further discloses a second redundant protection system with two IEDs acting in turn as main and auxiliary protection IEDs for two protected objects. Communication of process values can involve two distinct sensors on the same feeder, i.e. a first sensor for the main protection IED and a second sensor for the auxiliary protection IED.